Recently, as one of drought managements, seawater desalination plants to produce fresh water from seawater have been used. The seawater desalination plant supplies seawater to a reverse osmosis membrane, produces fresh water by filtering the seawater by the reverse osmosis membrane, and supplies the produced fresh water to customers through a distribution reservoir. Seawater desalination plants are known which control the pressure, flow rate and recovery rate (transmission rate) of seawater by pumps and valves. Japanese Patent No. 4341865 discloses a seawater desalination plant which controls the pressure of seawater applied to a reverse osmosis membrane by valves to thereby stabilize the quality of the filtrate water.
In general seawater desalination plants, operations thereof were performed so as to obtain a definite quantity of fresh water from a definite quantity of seawater as determined at the plant design stage. However, since the quality of seawater that is raw water changes from hour to hour, an electric power consumption rate required for obtaining fresh water is not constant. The filtration characteristic by a reverse osmosis membrane changes depending on the quality of seawater, for example. For this reason, if the seawater desalination plant is controlled so as to maintain the filtration characteristic to be constant, the electric power consumption rate of the electric power required for filtration also changes.
In a case where the temperature of seawater is high, since the viscosity coefficient of water is low, the transmission characteristic of a reverse osmosis membrane is good. In this case, since seawater passes through the reverse osmosis membrane at a relatively low pressure, the electric power required for pressurizing and supplying seawater is relatively small, and the required electric power consumption rate is small. On the other hand, in a case where the temperature of seawater is low, the required electric power consumption rate is large for the reason opposite to the above-described reason. In addition, in a case where the seawater salinity is high, the osmotic pressure difference between the seawater and the fresh water which is generated at the reverse osmosis membrane face is large, and the required electric power consumption rate is large.
For this reason, if the seawater desalination plant is operated so as to maintain definite fresh water production quantity, waste is incurred in the electric energy (electric power consumption rate) per day to obtain the target quantity of fresh water.
The problem to be solved by the present invention is to provide a seawater desalination plant system which is provided with a function to create a fresh water production quantity plan during a definite period so as to produce fresh water of production quantity required for the definite period with a minimum cost (electric power consumption rate), or a function to support the creation of the fresh water production quantity plan.